High pressure discharge lamps are often operated with a square wave current of a lower frequency in order to simulate direct current operation. This operating mode is often referred to as intermittent DC mode. The frequency of the low-frequency square wave current in such cases is a maximum of one power greater than the input AC mains frequency. In principle a high pressure discharge lamp is thus operated in dc mode, but the polarity of the lamp current is regularly reversed in order to place an even load on the lamp electrodes.
In such cases a power control system is generally implemented in order to operate the high pressure discharge lamp. With old lamps asymmetries in the lamp voltage sometimes occur, which can lead to undesired power fluctuations. These fluctuations can be suppressed by a correspondingly rapid control. Rapid control steps with a small time constant however have the disadvantage of a tendency for the high pressure discharge lamp to flicker during its operation. If the control has a very large time constant the tendency to flicker is small, but the asymmetries cannot be compensated for as a result of the large time constant of the control.
A large control time constant is to be understood here as a value which is much, i.e. by a power, greater than the time constant of the underlying operating frequency of the square wave lamp current.